[41]Revision, I, p. 76.
[41]Revision, I, p. 76.
The granulated hypostoma differs from that of Dalmanites proper in having the anterior margin strongly arched, the pointed wings almost on the median line of the hypostoma. There is one groove near the posterior border and the maculæ lie as two, narrow long, crescentic grooves close below the wings. The few sections give no clear idea of any structure.
The cephalic eyes are constructed on the same plan as in the previous species, the lenses being surrounded by a frame.
Dalm. vulgarisSalter.
Pl. III figs. 47-52.
The hypostoma has the common shield-like shape,fig. 51, anteriorly slightly arcuated, the short obtuse wings curvated towards the interior surface. It is finely granulated. The groove does not go so near the cuspidate posterior margin as in other species. The two maculæ,fig. 52, are narrow and elliptic, somewhat convex and lie in a little concavity, a narrow smooth space around is devoid of the granules, which cover the surface of the rest. They show absolutely no structure, excepting a few indistinct blackish spots.
The cephalic eyes have been often described and delineated. Most detailed are the descriptions and figures given bySalterin the Memoirs of the Geological Survey Dec. II. pl. 1. In the figure 4 a part of the surface is shown in well preserved state. He says that »the cornea is ... present and distinctly convex over each lens, the intermediate portions being ornamented with tubercles and granules». He thinks also that there are undeveloped or small lenses between the larger. His fig. 5 represents the frames around the lenses, these being probably lost. His explanation of fig. 6 I cannot understand. There are weathered or spoilt lenses figured.
There is no doubt that the granulated surface of the head continues between the more or less free lying lenses as shown here in a new figure taken from a specimenfrom Dudley,fig. 47. The granules of the surface are larger and more rare between the ocelli than for instance in Dalm. imbricatulus. Small lenses, undeveloped asSaltercalls them, may in fact only be such granules of somewhat larger size than usual. Lenses having the appearance as inSalter'sfig. 6, as if a covering was partially destroyed and the lens visible below it, I have also found in Swedish specimens, as represented infig. 47. In a vertical section,fig. 50, a delicate covering integument is seen and the oval lenses lie regularly with their frames around them quite as in Dalmanites obtusus and in horizontal sections,figs. 48, 49, there are also the same sort of black, irregular dots, being the sectioned tubes of the surface between the lenses. But in another feature there is great interest. Beneath each lens there are fascicules of tiny rods, twice as long as the lenses. They are represented in a vertical section infig. 50and in a tangential section infig. 49.
As our imperfect knowledge of so delicate anatomical structures in fossil crustaceans does not admit of secure comparisons with the visual organs of recent crustaceans, no suggestion can be given to interpret their nature. Probably they have no connection with the structure of the eyes and it is not even certain that they are of organic origin.
DysplanusBurm.
If there were no other characters to distinguish this genus from Illænus its strangely deviating hypostoma must do it. We have examined two species and in both the hypostoma is almost oval, evenly rounded both anteriorly and posteriorly. Near the anterior border two narrow, long hornlike wings project. They are flattened, thin and lamellar from the basis and end in a hollow, acuminated point.
D. centrotusDalm.
Pl. III figs. 53-56.
This species has the maculæ placed near the posterior border, in a semilunar flat, devoid of the terrace-lines. The maculæ are elongated tubercles and do not show any structure when sectioned. The cephalic eyes have a thin membrane covering the short cupshaped lenses, which, closely packed, in the transverse section give the usual polygonal mosaic of hexaeders. They have a radiate structure, when decayed.
D. ladogensisHolm.
Pl. III figs. 57, 58.
A specimen from Öland has the maculæ lying in the large field of terrace lines above the smooth zone at the posterior margin of the hypostoma. They are larger than in the former species, pearshaped, smooth and no structure visible.
EncrinurusEmmr.
I cannot find any former figure indicating the maculæ of the hypostoma in this genus than the only one ofAngelin, Pal. Scand. pl. IV fig. 6.
As to the shape of the singularly formed hypostoma in the three Swedish species Encr. punctatus, obtusus and lævis we have found the anterior border so characteristic in them that it is sufficient to distinguish them from each other and to annul the doubts concerning the distinction of Encr. lævis from Encr. punctatus. We therefore inpl. IV figs. 12-17give the anterior margins of these three species, along with the profiles.
Encrin. punctatusWahlenberg.
Pl. IV figs. 5-9, 12, 13.
The surface of the hypostoma is so finely and obscurely granulated that it seems to be almost smooth. It is in the shallow groove above the lamellar tongueshaped posterior border that the elongated tubercular macula are situated. They are prominent and in horizontal sections they have a surface mottled by irregular black and white specks, the whole surrounded by a white border,fig. 6. This border and the other surface is pierced by series of parallel slitlike pores,fig. 7, a feature peculiar to this genus. The vertical sections indicate a chain of indistinct prisms with a black, rounded central hollow between thick strata of clear whitish shell substance,figs. 8, 9. The shell substance on the sides is pierced by the straight, black tubes.
The cephalic eyes have the prisms indistinctly developed or badly preserved with a diameter of 0,04 millim. and height of 0,07 millim. In Encr. lævis the prisms are more distinct and larger with a diameter of 0,00 millim. (pl. IV figs. 10-11).
The maculæ are well developed, ovate, in E. lævis and obtusus and in the same position as in E. punctatus.
GriffithidesPortlock.
The hypostoma of Griff. globicepsPhill.shows according to the figure given inH. Woodward'swork pl. VI fig. 5 two somewhat obscure maculæ, but no mention is made of them in the description.
HarpesGoldf.
Harpes d'OrbignyanusBarr.Novák IITaf. I fig. 4, with small black maculæ. Harpes venulosusCorda.Novák II, Taf. I figs. 1, 2. Two narrow, horizontally placed maculæ.
HarpinaBarr.
H. primaBarr.Novák IITaf. I fig. 5. Two maculæ in continuation of a narrow groove, at the base of the central globosity.
HolmiaWalcott.
Holmia LundgreniMoberg, Sveriges äldsta trilobiter, Tafl. 14, fig. 10, 11, 12. The maculæ are much prominent and tubercular, elongate narrow, oblique but not mentioned in the description. Hypostoma probably belonging to the terrace-line group. As well seen in original specimens and in the figures in Dr.Holm'spaper on Olenellus (Holmia) Kjerulfi there are also in this species two distinct macula; near the posterior margin of the hypostoma, above the marginal groove.
HomalonotusKönig.
Maculæ have been previously observed in the following species.
Hom. delphinocephalusGreen,HallPal. N. York vol. II pl. 68 fig. 11, incomplete figure with two well marked tubercular maculæ. No description.
Hom. KnightiKönig.SalterII, pl. 12 fig. 10. Maculæ well expressed as oblique tubercles. On page 120 it is said »with a pair of lateral tubercles well developed (as in Asaphus ...)».
This species is identical withAngelin'sHomalon. rhinotropis, Pal. Sc., pl. XX fig. 1 e and his figure has two hollow maculæ seen from the inside of the hypostoma.
Homalonotus KnightiKönig(= Homal. rhinotropisAngelin).
Pl. IV figs. 20, 21.
The maculæ lie as oblong, smooth tubercles surrounded by the irregularly grown granules of the surface, just below the central globosity of the hypostoma. In a preparation from the interior surface of the macula, as seen in transmitted light,fig. 21, this is white with only a few grey spots and on the superior border black streaks, lying obliquely. The substance of the shell around the macula is perforated by the minutest pores amongst which a few larger are intermingled. The granules are also perforated as those in Calymmene and the tubes continue through the shell, visible by their black colour.
HysterolenusMoberg.
Hyst. TörnquistiMbg., En trilobit från Dictyograptusskiffern p. 320. In the descriptive letter-press, there is said, that in the exterior (lateral) parts of the anterior groove of the hypostoma a distinct tubercle is to be seen on each side. But none of the figures given, Pl. 17 figs. 6, 7, shows them or at least very indistinctly.
IllænusDalman.
It is remarkable, considering the great number of species in this almost exclusively Lower Silurian species, that there is none but a single species in which the maculæhitherto have been delineated. This is Illænus angustifrons var. depressesHolmI, pl. VIII fig. 18. A little, fragmentary hypostoma with globular maculæ.
Illænus ChironHolm.
Pl. IV figs. 22-25.
The hypostoma has the shape characteristic of all true Illæni, viz. a straight anterior margin with two large flat rectangular or nearly quadratic wings, a globular or spherical median field, forming the chief portion of the exterior surface, which is covered by some rare, fine, terrace lines. In some other Illæni the surface is smooth. At the posterior base of the median field the two maculæ are placed, tiny, oblong, but lying on the same level, at right angles to the median longitudinal axis of the hypostoma. They are covered with lenses of irregular and indistinct shapefig. 25. I do not think that they in this and the other species of Tiberius are lenses of the same conformation as in Bronteus, they are rather the tops of the subjacent lenses which are of the prismatic form. The cephalic eyes do not show any lenses on the outside. They have a narrow, opaque exterior integument, hiding hexaedral, thickly packed, straight prisms, forming a stratum in thickness of 0,2 millim.
The hypostoma consists of several thin layers of superposed calcite, which easily peal off, so that it is seldom that the true exterior sculptured surface remains.
Illænus EsmarkiSchloth.
Pl. IV figs. 26-33.
This the most common of all the Illæni has in several specimens shown the hypostoma with the maculæ in situ. The hypostoma is smooth excepting on the marginal ridges where there are some terrace lines. On the median axis of the central gibbosity (fig. 30) five faintly visible transverse folds are situated, a curious feature which we have not observed in any other trilobite. The maculæ are exceedingly small,fig. 31, elongate, spindleshaped and acuminated in both ends. On their surface there are accumulated several lenslike globules. As the horizontal sections show there are, however, no regular lenses, but a network of polygonal meshes with radii in the hollows,fig. 32. The perfect identity of this structure with that of certain states of the cephalic eyes will at once be perceived by comparing this figure withfigure 26representing the same sort of polyedric meshes, though a little more regular in the cephalic eye, where, however, the irregularity also prevails near the periphery of the eye. If we now compare the sections of the cephalic eye of Illænus Chiron and the section of the regular and nearly unchanged eyes of Ill. Esmarki with the second section of the latter, we cannot avoid the conclusion that we in those of Ill. Chiron,figs. 22, 23, and the first of Ill. Esmarki,fig. 27, see the primitive and intact state of the lenses and in the last sections of Ill. Esmarki,figs. 26, 28, the changed and deteriorated state of the same prismatic lenses.
The vertical section of the hypostomic eyesfig. 33shows in their present changed condition an evident longitudinal direction of the chief elements, quite as the lenses ofthe eyes. But in how far there ever has existed such prismatic lenses, now changed, we have no evidence definitely to ascertain, but the probability seems to be great that this was the case.
Illænus gigasHolm.
Pl. IV figs. 34-37.
The maculæ in this as well as the other Illæni are so faintly elevated and so inconspicuous that the greatest attention is needed to find them.
The maculæ are elongated, narrow, elliptic on the same level and parallel with the straight anterior margin of the hypostoma. A cast of the left macula is pitted by numerous marks of the lenticular globules of the surface,fig. 37. In the cephalic eye the prisms are seen translucent beneath the cornea. Near the superior border they are larger,fig. 34, almost the double size of those near the inferior border,fig. 35.
Illænus sphæricusHolm.
Pl. IV figs. 42, 43.
Comes near to the previous species. The globules on the surface of the elliptic maculæ are not, however, spread over the entire macula, leaving only a narrow border all around free.
The lenses in the cephalic eyesfig. 42are globular as seen on the surface, alike those of the maculæ.
Illænus RoemeriVollborth.
Pl. IV figs. 38-41.
The maculæ lie on elevated tubercles and are of a strange shape, being sharpely pointed outwards, rounded inwards. That infig. 39is still more peculiar with a narrow stripe on the interior and upper margin. The lenses are larger and more distinct than in any other species of this genus.
Beside the species now mentioned we have also observed hypostomas with maculæ in Illænus laticlaviusEichwaldfrom Estland, in Illænus LinnarssoniHolm, and in an unnamed species from Dalecarlia figured without macula by Dr.Holmin his memoir on the Illænidæ pl. III fig. 22. We have not seen any lenses on the maculæ of these three lenses.
LichasDalman.
We have not been able to find any notice about the exceptional maculæ of this genus. There is only a figure of the hypostoma of Lichas hylæusHallPal. of N. York vol. VII pl. XXV fig. 5 which would seem to show a pair of narrow, oblique maculæ, but these are quite without resemblance with the maculæ of the true Lichas.
The hypostoma of Lichas is of a characteristic type, broad, with the incised posterior margin and its two lappets reminding of Asaphus, with its large rounded, faintly elevated median part near the anterior margin reminding of Illænus, as also do the lenticles of the maculæ. The maculæ are of a minimal size, scarcely 1 millim. in diameter and thence easily overlooked.Pl. IV figs. 46, 48.
As the hypostomas in this genus generally are found detached it is often difficult to decide to which species they have belonged. So it is with that delineated inPl. IVfig. 44 natural size. The two maculæ are situated near the centre of the hypostoma, just below the grooves which surround the large, globular disc. They are sunk in a little cavity, rounded and entirely covered by globular lenses (pl. IV fig. 46).
In another detached hypostoma of an unknown species the macula is smaller and the few lenticular globules larger than in the other,fig. 48.
Lichas latifronsAngelin.
Pl. IV fig. 49.
The maculæ, seen in a thin section from the inside, are reticulate,fig. 49, or of a pattern exactly like that in the changed eyes or maculæ of Illænus and even Asaphus, and thus indicating that it was composed of short prismatic lenses. The vertical section of a macula of a specimen from another locality rather suggests the presence of large sphærical lenses than elongated prisms. The great thinness of the macula in contrast to the excessive thickness of the shell around the eye is seldom so evident as here.
A specimen of an unknown species, found detached, has the macula? unlike the other species, oblique with the inferior end directed inwards, placed on the top of a sort of a little elevation,fig. 47.
MegalaspidesBrögger.
This author has a figure of M. dalecarlicus in II pl. 1 fig. 19 and also of an unnamed species fig. 20, both showing globular maculæ. InHolm'soriginal description of his Megalaspis dalecarlicus (Trilobiten des Phyllograptusschiefers Dalecarliens figs. 8, 9) the same hypostoma is also figured. This genus seems to have a closer affinity with Ptychopyge than with Megalaspis.
MegalaspisAng.
The following figures of hypostomas with maculæ have been published.
Megal. limbatabyBröggerI, tab. XII, fig. 10, and in II pl. 2, fig. 22, somewhat differing from the first figure.
Meg. planilimbataBröggerII tab. 2 figs. 21, 21 a. Both figures incomplete and incorrect.
The hypostoma has a large central ovate gibbosity with the maculæ near its basis, surrounded by the winglike expanded limbus.
Megal. attenuataWahlenberg.
Pl. V figs. 1-6.
The long and narrow maculæ are crescent-shaped and project beyond the hypostoma supported on the superior surface of a sharp extenuated edge where they rest as on a shelf. They consist, as seen in vertical sections,figs. 5, 6, of alternating rows of light and black streaks, and in a horizontal section the spongious network appears, as it is found in Asaphus and others and thus indicating the tendency of this organ to assume a prismatic structure though undeveloped.
The cephalic eyes in M. attenuata form the most regular hexaedral prismsfig. 1 a, short and covered by a thin cornea. The border zone of the eyes has a structure exactly like that of the maculæ, the same as in the spongious zone of Asaphus.
Megal. limbataBoeck.
Pl. V fig. 7.
We give a new figure ofBrögger'soriginal specimens, as his figure 23, Tab. II is incomplete and incorrect. The maculæ are situated a little higher up on the sides of the central gibbosity.
Megal. planilimbataAng.
Pl. V fig. 8.
The maculæ are crescent-shaped and not so prominent as in Meg. attenuata and the visual spot in the same position as there.
To observe is that the general structure of the hypostomic shell in this and many other species is prismatic.
NileusDalman.
Hypostoma with maculæ figured in N. armadillo byAngelinpl. XVI fig. 5 c, andBr. IIpl. 3 fig. 40.
The hypostoma in this genus is transversal with large lateral expansions, the central convexity only faintly elevated and almost coherent with the posterior part. The maculæ lie exactly on the horizontal median line of the hypostoma. The anterior wings are well developed like slightly bent acuminated horns and the posterior wings are conical points. The terrace lines are transverse and a little wavy.
Nileus armadilloDalm.
Pl. V figs. 10-15.
There is some variability in the hypostoma of specimens from different localities. The most known and numerous specimens are from the renowned localities of Östergötland. They are decidedly transverse, the proportion of the hight to breadth is as 3 to 5 and the narrow border runs all round. In specimens from Kongslena, Vestergötland, the proportion is as 15 to 22, in specimens from Sandvik, Öland, again, we have 13 to 23 or 6 to 11. The maculæ,fig. 14, are oblong, inwardly pointed, quite smooth, with a little concavity in their centre, and their surface is slanting obliquely towards the surface of the hypostoma, almost as in Niobe. There is no trace of any structure.
The cephalic eyes are covered by a comparatively thick membrane, a direct continuation of the general test,fig. 12, hiding a row of prismatic lenses. In some specimens elongate, clear, crystalline rods continue downwards from the bases of the lenses and are probably nothing but inorganic crystals of calcareous spar,fig. 11.—The eyes are larger comparatively than in other species.
Nileus (Symphysurus) palpebrosusDalman.
Pl. V figs. 16-18.
As to its general form the hypostoma resembles that of the preceding species, but there is no border line, which is so characteristic to the former. The maculæ have the same form and are likewise situated on the median line. The cephalic eyes have prismatic lenses with much convex bases, in a transverse section they are hexaedral and of internal radiate structure.
NiobeAngelin.
The following figures have formerly been given of its hypostoma with maculæ.
N. emarginulaBr. IItab. 2 fig. 33.
N. explanataBr. IItab. 2 fig. 35.
N. frontalisBr. IIfig. 37.
N. insignisBr. Itab. IV fig. 1 d, 11 tab. 2 fig. 28.
N. lævicepsBr. IIpl. 2 fig. 34.
The maculæ are distinguished from all other sorts of maculæ, excepting those of Megalaspis, in being, as it were, shelved on a sort of support, formed through the bulging of the hypostomic surface just below them, one such console for each. Their surface lies consequently almost rectangularly to the adjoining surface of the hypostoma. They are white and smooth.
Niobe frontalisAng.
Pl. V figs. 19-21.
The pocketlike consoles are shorter and broader than in N. læviceps and the maculæ also larger and of an elliptic shape. In a horizontal section near the surface the macula has a speckled appearance of black dots, indicating an undeveloped prismatic structure, much more primitive than in the Asaphidæ.
Niobe lævicepsAng.
Pl. V figs. 22-26.
It differs from the preceding through longer consoles below the maculæ, which are oval and in a horizontal section exhibit the same spongious structure,pl. V fig. 25. The cephalic eyes consist of polyedric prisms, which through deterioration show a central cylindrical core surrounded by darkish matter like Asaphus. The passage from solid, homogenous prisms to changed ones is most evident in thefigure 22.
OgygiaBrongn.
Figures of the maculæ are found in
O. BuchiSalter, Mem. Geol. Surv., Dec. II pl. VI fig. 3 p. 2, where it is stated that »there are two transverse furrows near the apex, with compressed tubercles between them» There are two narrow crescentic maculæ and between them a little lower a longer, crescentic ridge, which may be the ridge that in other species connects the maculæ. Moreover, the shape of the hypostoma is not concordant with that of the other species. In Monogr. Brit Trilob. pl. 15 figs. 2, 3 there is only a reproduction of the former figure.
O. corndensisMurch.Salterin Monogr. pl. 16 fig. 10 interior side of the hypostoma with two lateral maculæ. In the description two pairs of furrows the uppermost is the maculæ.H. Wyatt-Edgell»On the Genera of Trilobites Asaphus and Ogygia and the Subgenus Ptychopyge» in Geol. Magaz. 1867, p. 14, 15 fig. 2 probably a cast with two oblique impressions of maculæ.
O. dilatata(Asaphus)Sarsvar. SarsiAng.Sarsin Isis 1835 p. 342, pl. IX fig. 11. The figure has two small maculæ near the posterior margin and the author says »mit einer starken Vertiefung oder Einschnitt an jeder Seite des Endes».Angelinpl. XLII fig. 1b.Brögger'sfig. 38 pl. 3, in II, collies near to the specimen, which is described below, also belonging to Dr.G. Holm.
O. dilatatavar.SarsiAng.
Pl. V figs. 27, 28.
The hypostoma is in so far deviating that the terrace lines have a nearly vertical direction and in this respect much resemble those of the Phillipsiæ and the Proeti. Themaculæ, near the exterior angles of the border groove are pear- or spoon-shaped with the apex directed downwards and outwards, and in consequence the longitudinal axis quite opposite to that of the usual direction. The surface is smooth, a little concave. No structure has been discovered. From the pointed apex a slightly elevated ridge runs along the posterior border of the hypostoma and joins the apex of the other macula.
ParadoxidesBrongn.
The oldest record of any hypostoma at all amongst the trilobites is that given byWahlenbergin his Petref. Suecana (1818) p. 37 Tab. 1, fig. 6, which he, however, considered as the head of the trilobite which he named Entomostracites bucephalus. Figures with maculæ have been given of
Par. DavidisSalterMem. Geol. Survey Dec. XI pl. X, fig. 3, a fine figure of a hypostoma with two large oblique maculæ.Linnarssonin »de undre Paradoxideslagren vid Andrarum» pl. II fig. 2 delineates a gigantic hypostoma with two crescentic tubercular maculæ, having 10 mms. in length.
Par. ForchhammeriAngelinPl. II fig. 3.Brögger, Paradoxidesskiffr. vid Krækling tab. II fig. 10 two marks somewhat different fromAngelin'sfigure.Linnarsson1. c. pl. I figs. 9, 10 two hypostomas with maculæ.
Par. Tessini, var.WahlenbergiAngel.Pl. I a, fig. 1 b.
Par. Tessini, var.oelandicusAng.Tab. I a, fig. 2 b.Angelin'sfigure of this hypostoma is incomplete in so far that he had not found the peculiar falciform horns on both sides of the posterior border. Pl. V, fig. 33.
Par. rugulosusCordahas, according toBrögger, Krækling, pl. II fig. 2 two long, straight callosities, probably corresponding to the maculæ in other Paradoxidæ.
Chr. Boeckgives in Mag. for Naturvidenskaberne Bd 8, 1828, in his paper on the Trilobites on the plate fig. 16 the hypostoma of a Bohemian species with well developed concave maculæ, probably a cast. He compares it with the Entom. bucephalus ofWahlenberg, but says he cannot explain its nature.
The maculæ of P. oelandicus, pl. V, fig. 34, are tubercles, oblong, smooth, and have invariably an oblong scar along the centre, where probably once a thinner membrane, as in the cephalic eyes of Calymmene, contained the lenses.
PelturaM. Edw.
Peltura scarabæoidesWahlenb.
Pl. III fig. 42.
We have given a figure of the hypostoma, showing two large, elongated tubercular maculæ or rather resemblances of such, as they are covered with terrace lines as well as the rest of the surface. They have, however, the same position as real maculæ might have, but are directed in a line parallel with the longitudinal axis of the hypostoma. The cephalic eyes have been described above atpage 29.
PhacopsEmmrich.
Maculæ figured of Phac. (Acaste) Downingiæ.SalterMonogr. pl. 2 fig. 34 b, p. 25 »high up on each side a small tubercle».
Phac. ranaHallPal. N. Y. vol. VII, pl. VIII A, fig. 18. Hypostoma with the large commalike maculæ high up.
Of the following species Ph. cephalotes and Ph. macrophthalmus belong to the genus Phacops proper, while Phac. quadrilineata, as well as Ph. Downingiæ are of a quite different generic type. The name Acaste proposed for them is not good asLeachin 1817 named a Cirrhiped as Acasta.
Ph. cephalotesBarr.
Pl. V figs. 35, 36.
The hypostoma is elongate, almost triangular, tricuspidate at the posterior border, a little above this there is a crescent-shaped ridge, and still a little higher above this and the median horizontal line, the two macula, as diminutive tubercles. Seen with magnifying powers they are slightly oblong or reniform.
Ph. macrophthalmusBurm.
Pl. VI figs. 1-9.
The triangular hypostoma has a little below the median horizontal diameter two shallow grooves, and a little above the same diameter the extraordinary minute maculæ may with some care be observed,figs. 8, 9. They are visible as a small bare, smooth spot amongst the curious elevated and incoherent terrace lines of the surface. The exceedingly small size of these maculæ naturally does not allow any research of their structure. They are rather in this species and many of the congeneric to be regarded as small ocelli. In some specimens these spots are quite rudimentary and are reduced to the smallest area possible. The anterior wings are of a peculiar earlike shape, finely striate, enlarged and flat near the hypostoma and ending in a little hollow horn (figs. 6, 7). A little below it, the posterior wing emerges as a small angular knob. The aggregate cephalic eyes are more distinct.
As the vertical section learn us the regular ovate lenses consist generally of clear, crystalline calcareous spar and are on the surface covered by an extremely thin membrane, not a common one for all, but a separate cornea for each lens, which envelopes its superior moiety and continues down as a thickened appendix between the lens and the interstitial test (figs. 3-5). In the horizontal sections it surrounds the lenses as an annular wall. Around some lenses there is as if still another ring lay outside this, but more indistinct, probably only clearer portions of the test surrounding them. The cheeks close to the lenses are indistinctly perforated by longitudinal tubes.
Phacops (Acaste) quadrilineataAngelin.
Pl. V figs. 37, 38.
We have not succeeded in finding any well preserved hypostoma.
The lenses of the cephalic eyes resemble much those of the preceding species though more elongate and ovate. They are covered by a membrane which surrounds their upper moiety and between the lenses joins the interstitial test. In the horizontal section these membranes surround the lenses as an annular wall.
PhaëtonidesBarr.
Ph. StokesiMurch.
Pl. VI fig. 11.
The hypostoma somewhat reminds of that of Calymmene, has projecting angles and a prominent knob below the straight anterior border. The two oblique and oblong macula are tubercular and placed near the lateral borders. This hypostoma is upon the whole much related to that of Phillipsia and Proetus.
PhillipsiaPortlock.
De Koninckfigured a hypostoma of Phillipsia in his »Description des Animaux Foss. de Belgique» as Cyclus Brongniartianus pl. LII fig. J andNovákin III figs. 6, 7 copied it and corrected the error. It is provided with two globular maculæ united by a curved ridge.
Phillipsia EichwaldiWoodw.NovákIII fig. 5 a hypostoma with globular maculæ. We have been able to examine three different, unnamed or undeterminated species, which we distinguish by numbering them.
Phill. No. 1from the Keokuk group of Crawfordsville, N. America. The hypostoma is broader than in the other species. The nearly horizontal elongated maculæ placed near the median line, close to the lateral margins.
Phill. No. 2(pl. VI, figs. 15-18) probably from the Carboniferous formation of Belgium is of a lengthened form. The elliptic maculæ in the groove, below the central elevation. They are smooth and surrounded by a flat border.
The cephalic eyes are much destroyed, but seem to have consisted of somewhat semi-prismatic lenses or prisms of the same type as Dysplanus.
Phillipsia No. 3(pl. VI figs. 12-14) from the Carboniferous strata of Beeren Eiland communicated by HrJ. G. Andersson. It comes near the preceding, is lengthened, but has broader and shorter anterior wings and the inferior surface just above the posterior border peculiarly pitted. The macula lying above this pitted field are ovate. The lenses of the cephalic eye are hemispheric on the surface.
PlatymetopusAngelin.
Pl. planifronsAngelin.
Pl. IV figs. 50, 51.
This genus may well be kept distinguished from Lichas in consequence of its peculiar hypostoma and its still more peculiar maculæ, not to remind of the other deviating features. The maculæ lie at the end of narrow, sigmoid grooves of their own above the long transverse groove, separating the posterior border from the anterior elevated moiety. They are elongated, curved and entirely covered with a great number of irregular lenticles, translucent through the thin covering membrane.
PliomeraAng.
Pliomera TörnquistiHolmin Trilobit. Dalecarliens p. 5 fig. 2.
While Pl. Fischeri and Pl. actinura not have any hypostomic maculæ, this species, which Dr.Holmwith some doubt joins in the same genus, has a hypostoma, which as to its general shape resembles that of both the mentioned species, evenly rounded at its posterior border, with a shallow groove encircling the central field. A little below the horizontal diameter of this the maculæ stretch obliquely inwards and downwards. They rest on a little elevation and are elongate, anteriorly acuminated. The figure onHolm'splate does not show the maculæ.
ProetusSteininger.
There are some indistinct indications of maculæ in several of the hypostomas figured inBarrande'sSystème pl. 15.
Proet. signatusLindstr., Gotl. Trilob. Pl. XV fig. 17, shows very distinctly the two maculæ.
Proet. concinnusDalm.
Pl. VI figs. 19-23.
In this, as well as in the few other species of Proetus which we have studied, there is that peculiarity of the surface of the hypostoma that two more or less distinct ridges form an acute angle just below the faintly arcuated anterior border. The terrace lines are exactly alike those of the Phillipsiæ, only visible on the central field and ceasing just above the maculæ, where there is a transverse smooth field. The posterior margin has two pair of very short points, one on each side. The macula are lengthened tubercles, somewhat bent and have an oblique position. They are surrounded by tiny prickles and as seen infig. 23the macula proper forms a white, elliptic spot on thetubercle. The cephalic eyes are lengthened prisms covered by a thick membrane. They are convex at their base and in a transverse section they show a radiate structure of their interior.
Proet. conspersusAngelin.
Pl. VI figs. 24-26.
We have figured a decorticated hypostoma with impressions of two reniform maculæ. The vertical sections of the cephalic eyes show a thin stratum where the lenses are much shorter than in the previous species and covered by a thin film of the common test. The same is also the case in a species which probably is Proet. verrucosus.
Proet. signatusLindström.
Pl. VI figs. 27-30.
On the lateral borders of the hypostoma there are short spines, two on each side, but they are placed differently on the specimens and even wanting in some. The elongated, elliptical maculæ bear in their lower end a white spot on which is seen a little cluster of a varying number of small, segregated ocelli, from three to five, according to the different specimens, like so many black points. The maculæ attain the exceptional length of 4,7 millim. in some specimens, but commonly only 2 millim. We have as yet not been able to detect these remarkable ocelli in any other species of Proetus.
PtychopariaCorda.
Walcottin »Palæontology of the Eureka District», gives on plate X fig. 21 a representation of a hypostoma which near the posterior margin shows two maculæ as narrow, crescentlike ridges and above them two longer and thicker ridges directed obliquely toward the central axis of the hypostoma.
PtychopygeAng.
Previous figures of hypostomas with maculæ are the following:
Pt. aciculataBr. II, pl. 1 fig. 13.
Pt. glabrataBr. II, pl. 1 fig. 14.
Pt. aciculataAng.
Pl. VI figs. 40, 41
The broad hypostoma bears the elongated, acuminated maculæ in an oblique direction, contrary to what is seen in most other species. The anterior end slopes inwards andthe lower end outwards, the surface, which in the cast drawn is surrounded by a marginal line, is directed sidewards against the broad lateral wings and is obscured by the declivity formed by the groove. The same disposition of the mature also occurs in Ptych. glabrata. These two species consequently in this respect differ essentially from the other four species which we have examined, but there is for the rest no reason to make these form different genera or subgenera.
Pt. angustifronsDalm.
Pl. VI figs. 31-36.
There are two varieties of hypostomas: one broadfig. 32and one more elongatedfig. 33. The maculæ having a convex surface lie at the inferior end of the lateral grooves prominent on a semicylindrical support, slightly inclining outwards. In the broad variety they are oblique (fig. 34) and in the elongated variety they are horizontal (fig. 35). In a vertical sectionfig. 36we have the same indications of a prismatic arrangement in the maculæ as in the Asaphi. The horizontal section does not represent so clear and distinct a reticulate structure as in the Asaphi, the prisms are much smaller.
The cephalic eyes are composed of closely packed hexaedral prisms, the gradual change from solid homogenous ones to those which are interiorly destroyed, only with the exterior form preserved is easily seen. The figure of a horizontal section (fig. 31) gives a good idea of the peculiar destruction of the interior of these prisms. In the vertical section the prisms are not discernible.
Pt. cinctaBrögger?according toFriedr. Schmidtin Mus. Holm.
Pl. VI figs. 37-39.
The hypostoma whichBröggerfigures (II, pl. I fig. 2a) as belonging to this species is completely identical with that of Pt. angustifrons, while the specimens whichFr. Schmidthas marked out as being the true Pt. cincta have a different hypostoma. The maculæ vary in the specimens, being in some placed horizontally or in a right angle to the longitudinal axis of the hypostoma, in others, again, a little obliquely outwards. They form prominent, free lying tubercles. It is from the superior face that the rudimentary lenses have left their traces, as seen in a longitudinal section (fig. 39). It is consequently this superior surface which is the macula proper.
Pt. glabrataAng.
Pl. VI fig. 42.
We have given a figure of the elliptic acuminate maculæ as seen on a cast of the inside of a hypostoma, the same which is the original to fig. 14 inBr. II.
SchmidtiaMarcou.
Moberghas in his paper on »Sveriges äldsta Trilobiter», pl. 15, fig. 6, drawn a hypostoma of Schmidtia Torelli with scars of maculæ, placed like that of the Paradoxidæ. There is no mention made of them in the descriptions.
TrochurusBeyrich.
Tr. pusillusAng.
Pl. IV figs. 52, 53.
The strange transverse, bipartite hypostoma carries the small, round, tuberculose maculæ on the superior moiety. As seen from the interior (fig. 53) they are obscurely maculate, but it is doubtful whether these black dots are derived from lenses, as the surrounding hypostomic surface also has the same mottled aspect.
Trochurussp.
Pl. IV figs. 54, 55.
This hypostoma belongs to an unknown species of Trochurus from the Southern Gotland (Burs). Its macula are placed almost as in the preceding species, but are much larger. They are surrounded by prominent margins. No lenses have been discovered on them.